1. Field of the Invention
The present invention relates generally to the field of telecommunications, and, more particularly, the present invention relates to a synchronization apparatus primarily used in the downlink (base station to mobile unit direction) of either a cellular mobile radio system or a so-called "last mile" radio access network for fixed connection telephony.
2. Description of the Related Art
The synchronization apparatus of the present invention employs the radio access method known as Code Division Multiple Access (CDMA) using Direct Sequence Spread Spectrum (DSSS). For convenience, the term "mobile" is used throughout this document to apply to the receiving end, however, this can also refer to the equipment at a customer's premises for the radio access network.
It is assumed that many signals are transmitted from the base station on a single carrier and that a common spread spectrum pilot signal is also transmitted to provide phase information, allowing coherent demodulation in the mobile receivers. Such a pilot signal also provides amplitude information for the different signal components received over the various radio paths, allowing efficient combining in a so-called "Rake" receiver.
DSSS radio links include a transmitter in which the data signal is spread by a spreading code, and a receiver in which the signal is despread by the same spreading code. Correct operation of the despreader requires synchronization of the code in the receiver with that generated in the transmitter, shifted appropriately by any delays in the signal path. In the case of a pilot signal assisted radio link, synchronization of the pilot and signal code are required. However, because of the fixed relationship between the pilot code and the signal code and because the pilot transmission is generally stronger than any individual signal transmission, synchronization is generally performed on the pilot alone.
Initial coarse synchronization is generally achieved either by examining many pilot code phases simultaneously with parallel hardware or by stepping a single receiver code phase until a correlation peak is found. Where minimum hardware complexity is desired, the latter approach is most frequently used. Once the approximate code phase has been found it is desirable to provide a means to achieve fine synchronization wherein the correlation for every bit is obtained at the peak value. The correlation shape is determined by the convolution of the transmitter filter shape with the receiver filter shape. For a frequency channelized system, as is generally necessary for the application cited, relatively sharp filtering will be applied and the correlation function will be a smoothly peaking function.
A conventional approach uses three correlators, an early, a late and a prompt correlator. Moreover, the signal must be sampled frequently enough to allow the different correlators to take the different timings simultaneously. Alternatively, the Tau dither loop could be used, but this has some disadvantages. In such systems, a dithering circuit is required in every receiver, and the signal code is never held on the prompt position but jitters about it.
The present invention has the following advantages: sampling once per chip only is required, and the timing of the sampling clock can be altered to achieve the fine control of the code timing. Only one pilot correlator and one signal correlator is needed. The average of the pilot code position is correct (prompt) and the signal code position is prompt so no additional loss arises from the jittering.
An object of the invention is to provide a method of permitting accurate fine synchronization of the code phase timing (to a fraction of a chip) with very low complexity in the mobile receiver.
Some of the complexity which would normally reside in the receiver is effectively transferred to the transmitter. However, this additional complexity applies only to the pilot generator in the transmitter.